1. Field of the Invention
The present invention relates to an oscillation circuit, and more particularly to an oscillation circuit suitable for use in a local oscillation of a narrow-band communication.
2. Description of the Related Art
A conventional oscillation circuit will be decribed with reference to FIG. 6. The oscillation circuit shown in FIG. 6 is a collector grounded type Colpitts oscillator, a collector of an oscillation transistor (hereinafter, simply referred to a transistor) 1 is connected to the ground (earth) through a grounding capacitor 2 with a low impedance in high frequency-wise. Also, feedback capacitors 3, 4 are connected between a base and an emitter, and between the emitter and the ground, respectively, and further, a coil 6 is connected between the base and the ground through a clap capacitor 5. As a result of this, a parallel resonance circuit 7 is formed with such a manner that two feedback capacitors 3, 4 are inserted through in a series, and also the coil 6 is connected in parallel to these two feedback capacitors 3, 4 connected in the series. To the coil 6, a correction capacitor 8 connected in series and a varactor diode 9 are connected in parallel, and to a cathode of this varactor diode 9, a tuning voltage is applied through a feeding resistor 10. Each of one ends of the feedback capacitor 4 and the coil 6, and an anode of the varactor diode 9 are directly connected to the ground, and connected to a collector of the transistor 1 through the ground and an earthed capactor 2 in high frequency-wise. Further, the clap capacitor 5 is the one which is not absolutely necessary.
To a base of the transistor 5, a bias voltage is provided by the base-biased resistors 12, 13 which are connected between the power supply terminal 11 and the ground, and also an emitter-biased resistor 14 is connected between the emitter and the ground. As the base-biased resistors 12, 13, usually the ones having the resistance value of a few Kilo-ohms are used, but as the emitter-biased resistor 14, the one having a resistance value of 100 to 200 ohms is used in the oscillation circuit which is driven by a low voltage.
The oscillation circuit with the above described structure is a most general one, and even though a detail description of its operation will be omitted, a parallel circuit 15 composed of the clap capacitor 5, the coil 6, the correction capacitor 8, and the varactor diode 9 turns to be an inductive as a whole, and by being connected in parallel to the feedback capacitors 3, 4 which are connected in series, a parallel resonance circuit 7 is configured between the collector and the base of the transistor 1, and an oscillation is hold from the fact that both ends of the parallel resonance circuit 7, that is, the collector and the base of the transistor 1 will become in a relation of an opposite phase by taking the connecting point of the feedback capacitors 3, 4, i.e., the emitter of the transistor 1 as a middle point.
This kind of oscillation circuit is widely used as the VCO (Voltage Control Oscillator) which tried to make it miniaturizing, electric power-saving, drive voltage-reducing, as for use in a portable telephone, recently, but accompanying with it, the C/N (Carrier v.s. Noise ratio) which is an important performance of the oscillation circuit is decreasing. The main reason for that is a decrease of the non-load Q as a result of a miniaturization, and further, a decrease of the Q of the parallel resonance ciruit 7 according to a bias resistance of the transistor 1, and more particularly, to the emitter-biased resistor 14 of a low resistance value. The emitter-biased resistor 14 is connected to the feedback capacitor 4 in parallel, and upon receiving a resonance voltage generated on both ends of the parallel resonance circuit 7 a portion of a resonance current flows, and upon consuming an electric power it will reduce the Q of the parallel resonance, thereby reducing the C/N. Smaller the resistance value of the emitter-biased resistor 14, larger the reductions of the Q and C/N. Then, since more voltage-reducing the drive voltage of the oscillation circut, the resistance value of the emitter biased resistor 14 has to be made smaller (in order to secure a predetermined voltage between the collector, emitter of the transistor 1), the reduction of the Q and the reduction of the C/N have been become very important problems.